Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Optical channel monitors are used in optical communications to monitor various characteristics of individual wavelength channels throughout a network. In wavelength division multiplexed (WDM) systems, multiple channels having different wavelengths are transmitted as a single combined signal. However, there are various optical phenomena that effect optical beams on a wavelength-dependent basis. Such effects include chromatic dispersion and wavelength dependent loss in optical materials. Therefore, it is beneficial to monitor the characteristics of each channel as it propagates through the optical system.
Traditional optical channel monitors can provide an inventory of incoming and outgoing channels and some devices can report on each channel's power and peak central wavelength. Some optical channel monitors include diffraction gratings for spatially separating individual wavelength channels from a WDM signal. Conventional channel monitors use imaging systems having a controlling lens to image a beam at a particular focal point. While these systems are relatively simple in design, the controlling lens necessarily has a large focal length in order to provide good spectral performance. As the scale size of the overall system scales roughly with the focal length, optical channel monitors implementing imaging systems are typically large in size. This is often disadvantageous in optical systems where scale size is an important consideration.
US Patent Application Publication 2009/0303562 (Koeppen et al.) entitled “High-Resolution Spectrally Adjustable Filter” discloses an optical channel monitor including a tiltable reflector such as a MEMS mirror and a diffraction grating for angularly separating individual wavelength channels. This device implements a non-imaging system wherein the optical beam remains essentially collimated throughout the system and is not focused at a point. Koeppen et al provides for a smaller scale size device as there is no focal length limitation to the location of the optical elements. However, such a non-imaging system substantially increases the complexity of design, particularly when multiple input/outputs are used. Furthermore, in this arrangement the size of the tiltable reflector and diffraction grating scales proportional to the number of inputs and outputs in the system.